Conventional vehicle body panels are formed by an outer steel panel and an inner steel panel that are welded together. For example, an inner roof panel may include integrally formed reinforcing sections or reinforcements may be welded to the inner roof panel to provide added strength in particular locations. Steel roofs are strong but relatively heavy and manufacturing processes involved are labor-intensive.
Recently, injection molded vehicle body panels have been developed. For example, an outer roof panel and an inner roof panel may be separately injection molded and then bonded together in a secondary operation. Fasteners may also be used to supplement bonding the inner and outer roof panels together. This process is slow and labor-intensive, requiring many steps to form the roof and then assemble the roof panels together. In particular, the inner roof panel must be designed with reinforcing ribs and gusset walls to provide strength. One problem associated with injection molding an outer roof panel is that surface imperfections caused by shrink marks are created in the injection molding die. An inherent problem for large, relatively thin injection molded panels that are used for vehicle applications is the tendency of the panels to distort or vibrate when air flow and air pressure differentials are applied to the moving vehicle.
Rotational casting is a plastic processing technique that has been proposed for high strength applications. However, rotational casting of plastics results in a relatively heavy finished product that is expensive and may not always meet close tolerance requirements.
Blow molding is a efficient and economical plastic processing technique that is most commonly associated with forming PET beverage containers. One problem associated with blow molding is that it is difficult to maintain close tolerances. While it is possible to form sharp edges and corners by blow molding, it is difficult to consistently form these design elements. Another problem with blow molding techniques is the difficulty in maintaining consistent wall thickness in larger parts.
These problems encountered by prior art vehicle body panel designs and available plastic processing techniques are addressed by the present invention as summarized below.